1. Field of the Invention
The present invention relates to a technique of controlling the amount of laser light used for exposure.
2. Description of the Background Art
A CTP (computer-to-plate) system for directly forming an image onto a printing plate includes a conventionally well-known CTP system that uses a laser diode (LD) in an exposure head (light source). In this CTP system, a calibration sensor is arranged in front of the LD and the amount of laser light is measured by the calibration sensor. Based on the amount thereby obtained, the value of a current to flow in the LD is controlled so that laser light can be output with a predetermined amount of light. A calibration process using this calibration sensor is normally performed each time a control board for the LD or the LD itself is changed. The calibration process is also generally performed at regular intervals in order to see the presence or absence of the degeneration of the LD.
A type of exposure for the CTP system includes a widely applied multi-beam type in which light beams are simultaneously emitted from a plurality of laser diodes to perform exposure. As an example, a publicly known CTP system comprises an exposure head with a plurality of laser diodes arranged in a linked manner in one direction, and a lens for focusing laser beams emitted from the plurality of laser diodes onto an exposure position (see for example U.S. Pat. No. 6,249,306). In another publicly known CTP system, a plurality of laser diodes are spaced apart at certain intervals, and laser beams emitted from the respective laser diodes are directly and concurrently applied to partial areas to thereby expose an entire area (see for example Japanese Patent Application Laid-Open No. 2003-89180).
An LED lighting system capable of easily being mounted or being changed to another is also publicly known (see for example Japanese Patent Application Laid-Open No. 2005-317586).
In the CTP system disclosed in U.S. Pat. No. 6,249,306, laser diodes are concentrated in one place. So a calibration sensor may be arranged in a position corresponding to the location of these laser diodes.
In contrast, in the CTP system disclosed in Japanese Patent Application Laid-Open No. 2003-89180, a plurality of laser diodes are discretely arranged. This requires a calibration sensor to be arranged in front of each laser diode at least at the time of execution of calibration. As an example, if one calibration sensor is intended to be responsible for the calibration of all the laser diodes, a mechanism for moving this calibration sensor is required, leading to the increase in system cost of the CTP system. The provision of calibration sensors in a corresponding relationship with all the laser diodes also naturally results in the increase in system cost.
After all, the larger number of laser diodes results in a longer period of time required for calibration. From a point of view of enhancing the operating rate of a system, calibration itself is desirably as simple as possible.